As shown in FIG. 4, a conventional speaker includes voice coil unit 2a, magnetic circuit 1a, diaphragm 3a and frame 6a. Voice coil unit 2a is movably disposed in magnetic circuit 1a. Diaphragm 3a is coupled to voice coil unit 2a at its inner peripheral end and coupled to frame 6a via edge 4a at its outer peripheral end. Furthermore, voice coil unit 2a and frame 6a are coupled to each other by damper 6a. 
Conventional damper 6a has a corrugated structure formed in a corrugated sheet with a material such as urethane, foamed rubber, SBR rubber, close, and the like. Such a corrugated structure secures a predetermined elastic modulus and suppresses the rolling occurring when voice coil unit 2a is driven. The above-mentioned conventional speaker is disclosed in, for example, Japanese Patent Unexamined Publication No. 11-150791.
Since damper 6a is formed in a corrugated structure, when the amount of amplitude of voice coil unit 2a is small, damper 6a is not loaded by the amplitude of voice coil unit 2a. However, as the amount of amplitude of voice coil unit 2a is increased, damper 6a is largely loaded by the amplitude of voice coil unit 2a. Therefore, the power linearity of the speaker shows nonlinearity, resulting in the generation of distortion of the speaker.